# std::ranges::generate

From cppreference.com
< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::input_or_output_iterator O, std::sentinel_for<O> S, std::copy_constructible F > requires std::invocable<F&> && std::indirectly_writable<O, std::invoke_result_t<F&>> constexpr O generate( O first, S last, F gen ); | (1) | (since C++20)
template< class R, std::copy_constructible F > requires std::invocable<F&> && ranges::output_range<R, std::invoke_result_t<F&>> constexpr ranges::borrowed_iterator_t<R> generate( R&& r, F gen ); | (2) | (since C++20)

1) Assigns the result of successive invocations of the function object gen to each element in the range `[`first`,` last`)`.
2) Same as (1), but uses r as the range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first, last | - | the range of elements to modify
r | - | the range of elements to modify
gen | - | the generator function object

### Return value

An output iterator that compares equal to last.

### Complexity

Exactly ranges::distance(first, last) invocations of gen() and assignments.

### Possible implementation

    struct generate_fn
    {
        template<std::input_or_output_iterator O, std::sentinel_for<O> S,
                 std::copy_constructible F>
        requires std::invocable<F&> && std::indirectly_writable<O, std::invoke_result_t<F&>>
        constexpr O operator()(O first, S last, F gen) const
        {
            for (; first != last; *first = std::invoke(gen), ++first)
            {}
            return first;
        }

        template<class R, std::copy_constructible F>
        requires std::invocable<F&> && ranges::output_range<R, std::invoke_result_t<F&>>
        constexpr ranges::borrowed_iterator_t<R> operator()(R&& r, F gen) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), std::move(gen));
        }
    };

    inline constexpr generate_fn generate {};

### Example

Run this code

    #include <algorithm>
    #include <array>
    #include <iostream>
    #include <random>
    #include <string_view>

    auto dice()
    {
        static std::uniform_int_distribution<int> distr{1, 6};
        static std::random_device device;
        static std::mt19937 engine {device()};
        return distr(engine);
    }

    void iota(auto& r, int init)
    {
        std::ranges::generate(r, [init] mutable { return init++; });
    }

    void print(std::string_view comment, const auto& v)
    {
        for (std::cout << comment; int i : v)
            std::cout << i << ' ';
        std::cout << '\n';
    }

    int main()
    {
        std::array<int, 8> v;

        std::ranges::generate(v.begin(), v.end(), dice);
        print("dice: ", v);
        std::ranges::generate(v, dice);
        print("dice: ", v);

        iota(v, 1);
        print("iota: ", v);
    }

Possible output:

    dice: 4 3 1 6 6 4 5 5
    dice: 4 2 5 3 6 2 6 2
    iota: 1 2 3 4 5 6 7 8

### See also

ranges::generate_n (C++20) | saves the result of N applications of a function (algorithm function object)
ranges::fill (C++20) | assigns a range of elements a certain value (algorithm function object)
ranges::fill_n (C++20) | assigns a value to a number of elements (algorithm function object)
ranges::transform (C++20) | applies a function to a range of elements (algorithm function object)
ranges::generate_random (C++26) | fills a range with random numbers from a uniform random bit generator (algorithm function object)
generate | assigns the results of successive function calls to every element in a range (function template)

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